The invention relates to a semiconductor laser having a semiconductor body comprising a first passive semiconductor layer of a first conductivity type, an active semiconductor layer present thereon, a second passive semiconductor layer of the second opposite conductivity type present on the active layer, a strip-shaped semiconductor contact layer of the second conductivity type present on the second passive layer, and contact means to apply a voltage in the forward direction between the contact layer and the first passive layer so as to generate coherent electro-magnetic radiation in a strip-shaped part of the active layer underlying the contact layer within a resonator, the passive layers having a smaller refractive index for the generated radiation than the active layer, a doped surface zone of the second conductivity type being present which extends over at least a part of the thickness of the contact layer and beside the contact layer extends in the second passive layer and has a higher doping concentration than the second passive layer.
The invention furthermore relates to a method of manufacturing the semiconductor laser.
A semiconductor laser of the kind described is known from the article by Bouley et al., in IEEE Journal of Quantum Electronics, QE 15, 1979, pp. 767-771. This article describes a laser structure of the type having a double hetero junction, with an active layer of p-type gallium arsenide which is present between passive n-type and p-type layers of gallium aluminum arsenide on the last of which a strip-shaped contact layer of p-type gallium arsenide is provided. The active region in which the radiation is generated and propagated is restricted in the lateral direction, that is to say in a direction parallel to the surface and perpendicular to the longitudinal direction of the strip-shaped contact layer, by a step in the refractive index which is caused by a zinc diffusion which extends over a part of the thickness of the contact layer, and which beside the contact layer penetrates through the p-type gallium aluminum arsenide layer and through the active layer into the n-type gallium aluminum arsenide layer. Furthermore, the current of the active region is laterally restricted by an electrically substantially insulating region which is obtained by proton bombardment and which also extends into the n-type passive layer.
With the known semiconductor laser described above a good linearity of radiation intensity versus current intensity is obtained, while good transversal and longitudinal radiation mode stability is also obtained.
A disadvantage of this laser structure, however, is that the zinc diffusion should be carried out through the active layer. As a result of this, defects in the crystal structure can easily arise in the active layer, which defects may afterwards give rise to degradation and consequent oscillations and hence to a shortening of the laser lifetime.
Furthermore, inter alia as a result of the comparatively large step in the refractive index of the active layer in the lateral direction, which in the laser described is of the order of 0.01, it has been found that higher order transversal radiation modes can nevertheless occur rather easily in the case of small deviations in, for example, the geometry of the layer structure or the current density. Finally, the leakage current on either side of the strip-shaped contact layer via the diffusion is rather large, which necessitates current restriction by means of a proton bombardment in this known laser, which means an extra complication.